This application claims priority benefit of Japanese Application 11-00 1994 filed Jan. 7, 1999.
1. Field of the Invention
The present invention relates to a structure of a body enabling the body reduce a fluid frictional resistivity generated when the body relatively moves against the fluid.
2. Brief Description of the Related Art
As a conventional structure of a body for reducing a fluid frictional resistivity, structures with rib-lets formed longitudinally on bodies, for example, on outer surface of ships or on inner walls of pipelines in order to reduce drag force due to a turbulent flow of the fluid, have been employed.
These rib-lets are aimed at reducing the fluid frictional resistivity affecting the surface by changing artificially a shape of a fluid velocity distribution inevitably generated due to an effect of a viscosity of the fluid in a boundary layer in the vicinity of the surface of the body.
Since the above-mentioned rib-lets, however, can not avoid non-slip flows relating the fluid velocity distribution on the surface of the body, there is a problem that sufficient reduction of the fluid frictional resistance is not attained.
The present invention is carried out to eliminate the above-mentioned conventional problem.
As shown in FIG. 1, a fluid frictional resistivity generated on a flat surface of body B is proportional to a frictional stress (xcfx84) defined by a undermentioned equation where a (du/dy), (which can take any value, i.e. positive, zero or negative value) a linear gradient of a velocity distribution (u) of the fluid flowing in the direction (x) along the vertical direction (y) on the surface S of the body B, is multiplied by a viscosity coefficient (xcexc) of the fluid.
xcfx84=xcexc(du/dy)|y=0
Therefore the fluid frictional resistivity of the body B can be reduced when the absolute value of the linear gradient (du/dy) is reduced or when the value of the gradient can be reversed from positive as shown in FIG. 1 to negative.
Accordingly, either by reducing differences of velocity components in the vicinity of the surface S of the body B to nearly zero, or by converting the value of the linear gradient (du/dy) from positive to negative, the fluid friction resistivity can be reduced.
For that purpose, in the present invention, grooves M are formed on the surface S of the body B so as to lead the fluid flowing along the surface S of the body B into the grooves and then to turn the flowing direction to the moving direction of the body, as shown in FIG. 2.
Thus, as shown in FIG. 2 at the bottom area of groove M reversed flows R of the fluid are generated by turning the flowing direction of the fluid and consequently the linear gradient of the fluid velocity along the flowing direction becomes negative. In addition, an absolute value of the linear gradient of the fluid in the area of grooves M can be decreased.
In the present invention grooves M are formed on the entire surface or on portions of the surface. In some cases long grooves can be formed continuously, while in other cases short ones intermittently or any combination of the long and short ones can be applicable.
Preferably the direction of the grooves M should be arranged in a way to cross the flowing direction of the fluid.
Grooves M with any cross-section are acceptable, provided that the cross-section can be effective in reducing the above-mentioned fluid frictional resistance. Examples of cross-sections are illustrated in FIGS. 3 to 8. In each figure, arrow mark represents the flowing direction of the fluid. In some cases the fluid flows parallel to the surface of the body, while in other cases it flows diagonally (in a crossing direction) to the surface.
A groove M1 in FIG. 3 has a rectangular cross-section, a groove M2 in FIG. 4 has a rectangular cross-section with a projection H1 on a downstream edge and a groove M3 in FIG. 5 has a rectangular cross-section with projections H2 on upstream and downstream edges. A groove M4 in FIG. 6 has a U-shaped cross-section with curved bottom corners. A groove M5 in FIG. 7 has an elliptical cross-section while a groove M6 in FIG. 8 has a circular cross-section.
Any kind of surface geometry such as flat, curved or corrugated etc. can be selected arbitrarily for the surface of the body B where grooves M are to be formed. A body with either a smooth or a rough surface can be employable in the present invention.
As a material for the body B, rigid, soft or elastic one can be employable. Any kind of material can be employable, as far as grooves M can be formed on the surface of it.